The present invention generally relates to a planar inductor on a semiconductor device.
A conventional planar inductor on a semiconductor chip or integrated circuit is described.
FIG. 1 illustrates a conventional multi-turn planar inductor.
As shown in the figure, a planar inductor 100 includes a substrate 102, a dummy metal fill 104, an input 106, a crossover 108, a crossover 110, a cross-under 112, a cross-under 114, an output 116, a trace portion 118, a trace portion 120, a trace portion 122, a trace portion 124 and a trace portion 126.
Input 106 provides a connection to Trace portion 118. Trace portion 118 is disposed between input 106 and crossover 108. Trace portion 120 is disposed between crossover 108 to cross-under 112. Trace portion 122 is disposed between cross-under 112 and crossover 110. Trace portion 124 is disposed between crossover 110 and cross-under 114. Trace portion 126 is disposed between cross-under 114 and output 116.
In operation, a current is transmitted through planar inductor 100 causing a magnetic field to form perpendicular to planar inductor 100. Current enters at input 106, flows through trace portions of planar inductor 100, and exits at output 116. Trace portions of planar inductor 100 in conjunction with crossover 108, crossover 110, cross-under 112, and cross-under 114 allow multiple levels of trace portions to function as a coiled wire.
As current flows through the trace portions of planar inductor 100, the associated magnetic field generates additional electromagnetic fields, which will be described in greater detail with reference to FIG. 2.
FIG. 2 illustrates a partial cross sectional oblique view of a conventional planar inductor 200.
As shown in the figure, conventional planar inductor 200 includes a semiconducting substrate 202 and a trace 204. Trace 204 includes a trace portion 206, a trace portion 208 and a trace portion 210. Input and output ports (not shown) are parallel to trace portion 206 and trace portion 210 and are perpendicular to trace portion 208. A positive charge 212, a negative charge 214, an image current 216, an electric field 218, an electric field 220, a current 222 and a magnetic flux 223 are additionally included in the figure.
In operation, current 222 enters at input (not shown) at proximal end of trace portion 206, travels from trace portion 206 to trace portion 208 and exits at output (not shown) at proximal end of trace portion 210. Current 222, flowing through trace 204, will generate a magnetic flux 223 between trace portion 206 and trace portion 210, which flows in au inward direction toward substrate 202. According to Lenz's law magnetic flux 223 causes circular eddy currents in substrate 202 that generate a magnetic flux (not shown) in a direction opposite to that of magnetic flux 223. Since the substrate is not extremely conductive and has a relatively small resistivity, the eddy current losses are small.
Positive charge 212 and negative charge 210 are illustrated as a current differential at a time t0 in order to show a direction of current 222. Positive charge 212 generates a spherically radiating electric field. The portion of the spherically radiating electric field, which is of interest, is that which is directed toward substrate 202, and is indicated as electric field 218. Electric field 218 then induces a negative charge on substrate 202.
Similarly, negative charge 214 additionally generates a spherically radiating electric field in the opposite direction of the spherically radiating electric field associated with positive charge 212. The portion of the spherically radiating electric field of negative charge 214, which is of interest, is that which is directed from substrate 202, and is indicated as electric field 220. Electric field 220 then induces a positive charge on substrate 202.
In this manner, the positive charge induced by electric field 220 and the negative charge induced by electric field 218 create image current 216, flowing in an opposite direction to current 222 within substrate 202. Image current 216 is smaller in magnitude than current 222 and rotates in a direction opposite to that of current 222 within trace 204.
For purposes of discussion, consider the example where the resistance of substrate 202 is around 12.5 Ωcm. This is not too high or too low. Since substrate 202 is not extremely conductive, the eddy current losses will be negligible as described earlier. Image current 216, when flowing in substrate 202, leads to resistive dissipation losses (I2R loss). Since Q is a function of the ratio of energy stored in magnetic flux to the energy dissipated, the resistive loss from substrate 202 leads to Q degradation. In order to minimize the substrate loss due to image currents, substrate 200 needs to be made highly conductive or highly resistive.
The quality factor (or Q factor) of an inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency. The higher the Q factor of the inductor, the closer it approaches the behavior of an ideal, lossless, inductor. The inductive reactance of an inductor is based on the overall generated magnetic flux. In the case of planar inductor 200, the overall magnetic flux is a combination of-magnetic flux 223 generated by current 222 and the magnetic flux (not shown) generated by image current 216 in substrate 202. For example, a conventional planar inductor similar to planar inductor 200 may provide a Q factor in the range of 10 to 16.
In order to make the substrate highly conductive one conventional method uses a blanket metal shield, and will now be explained with reference to FIGS. 3A-B.
FIG. 3A illustrates a top view of a conventional planar inductor 300.
As shown in the figure, conventional planar inductor 300 includes a blanket metal shield 302, a substantially rectangular trace 304, and a substrate (not shown). Trace 304 includes a trace portion 306, a trace portion 308, a trace portion 310, a trace portion 312, a trace portion 314, a trace portion 316, and a trace portion 318.
Blanket metal shield 302 is disposed between trace 304 and the substrate (not shown). Trace portions 306 is parallel to trace portion 318, whereas trace portion 310 is parallel to trace portion 314. Trace portions 308 and 316 are parallel to trace portion 312. Trace portions 310 and 314 are perpendicular to trace portions 308, 312, and 316.
In operation, current flows through trace portions of planar inductor 304. Trace portion 314 and trace portion 316 serve as input and output. Blanket metal shield 302 forms a conductive layer disposed between substrate (not shown) and trace 304. In FIG. 2, an inductor with trace disposed on semiconducting substrate, without shield, induces an image current within substrate. The resistive losses in the substrate (not shown), due to the flow of this image current, cause the Q to degrade. FIG. 3B illustrates a partial cross sectional oblique view of conventional planar inductor 300. This figure describes certain aspects of electrical and magnetic behavior of planar inductor 300.
As shown in FIG. 3B, conventional planar inductor 300 includes substrate 202, trace 304, and blanket metal shield 302.
Additionally shown in the figure are positive charge 320, negative charge 322, electric field 324, electric field 326, current 328, eddy currents 330 and a magnetic flux 332.
In operation, current 328 enters through input (not shown) at proximal end of trace portion 310, travels through trace portion 310 to trace portion 312 and exits at output (not shown) at proximal end of trace portion 314. Current 328, traveling in a circular manner, will generate magnetic flux 332 that is between trace portion 310 and trace portion 314, which flows in an inward direction toward substrate 202.
Similarly to planar inductor 200 of FIG. 2, a positive charge on trace portion 310 of planar inductor 300 will induce a negative charge on blanket metal shield 302. Similarly, a negative charge on trace portion 314 will induce a positive charge on blanket metal shield 302. In contrast with planar inductor 200, because blanket metal shield 302 is a metal shield, it conducts the induced positive and negative charges induced thereon. These charges cause an image current to flow in a direction opposite to the current in trace 304. Since blanket metal shield 302 is highly conductive, the image current leads to negligible I2R resistive losses. Disposing an intermediate conductive shield between trace and substrate reduces Q degradation due to resistive losses due to image current within substrate.
However, magnetic flux 332 generates eddy currents 330 within blanket metal shield 302. Since blanket metal shield 302 is highly conductive the eddy currents 330 are large and comparable to current 328. These eddy currents 330 generate associated magnetic flux that are opposite in direction and comparable in magnitude, and which counter magnetic flux 332. In the case of planar inductor 300, the overall magnetic flux is a sum of magnetic flux 332 generated by current 328 and a large opposite magnetic flux (not shown) induced by eddy currents in blanket metal shield 302. Accordingly, the Q factor is severely attenuated by the presence of eddy currents within blanket shield.
In planar inductor 200 of FIG. 2, the Q factor is diminished as a result of the image currents generated in the lossy substrate 200. In planar inductor 300 of FIG. 3B, blanket metal shield 302 prevents image currents from forming within lossy substrate 202. However, in planar inductor 300 of FIG. 3B, the Q factor is diminished as a result of large eddy currents generated in blanket metal shield 302.
A metal shield disposed between trace and substrate eliminates image currents within substrate but induces eddy currents due to charges generated within conductive shield material. Eddy currents exist more readily within a conductive material where charges can move more freely than in a semiconductor or insulator material. Conventional efforts attempt to reduce losses in Q factor of planar inductor by using a patterned metal ground shield to minimize eddy currents in the conductive shield and minimize image currents in the lossy silicon substrate. This conventional slotted planar inductor will now be explained with reference to FIG. 4.
FIG. 4 illustrates a top view of a conventional planar inductor 400.
As shown in the figure, conventional planar inductor 400 includes a substrate (not shown), trace 304 and a randomly-traced metal shield 402. Metal shield 402 includes a shield portion 404, a shield portion 406, a shield portion 408, and a shield portion 410. Metal shield 402 is disposed between a substrate (not shown) and trace 304.
Shield portion 404 includes a plurality of parallel traces arranged to resemble stripes. The traces are very closely spaced and have different lengths, a sample of which is indicated as a trace 412. Shield portion 406 includes a plurality of parallel traces arranged to resemble stripes. The traces are very closely spaced and have different lengths, a sample of which is indicated as a trace 414. Shield portion 408 includes a plurality of parallel traces arranged to resemble stripes. The traces are very closely spaced and have different lengths, a sample of which is indicated as a trace 416. Shield portion 410 includes a plurality of parallel traces arranged, to resemble stripes. The traces are very closely spaced and have different lengths, a sample of which is indicated as a trace 418.
The parallel traces of shield portion 404 are arranged such that the length of each trace is parallel with the length of each of parallel traces of shield portion 408. The parallel traces of shield portion 404 are additionally arranged such that the length of each trace is perpendicular with the length of each of parallel traces of shield portion 418. The parallel traces of shield portion 404 are additionally arranged, such that the length of each trace is perpendicular with the length of each of parallel traces of shield portion 406.
Metal shield 402 is disposed between trace 304 and substrate (not shown).
A portion, indicated by dotted trapezoid 420, of the plurality of parallel traces of shield portion 408 are perpendicular to trace portion 314. A portion, indicated by dotted five-sided object 422, of the plurality of parallel traces of shield portion 406 are parallel to trace portion 314. Further, a portion, indicated by dotted five-sided object 424, of the plurality of parallel traces of shield portion 410 are parallel to trace portion 314.
A portion, indicated by dotted trapezoid 426, of the plurality of parallel traces of shield portion 404 are perpendicular to trace portion 310. A portion, indicated by dotted five-sided object 430, of the plurality of parallel traces of shield portion 406 are parallel to trace portion 310. Further, a portion, indicated by dotted five-sided object 428, of the plurality of parallel traces of shield portion 410 are parallel to trace portion 310.
In operation, trace 304 in FIG. 4 performs a function equivalent to that of trace 304 in FIG. 3A. A shield 402 comprises distinct and separate conductive traces so that eddy currents are constrained within each trace and loss in Q factor is minimized.
Importantly, metal shield 402 in FIG. 4 contains very closely spaced traces that are randomly sized and randomly placed. Trace 412 is representative of traces in metal shield 402. The trace pattern (i.e., the width of the metal shield traces, the spacing between the metal shield traces, the length of the metal shield traced and the symmetry and orientation w.r.t main inductor shield) of shield portions 404, shield portion 406, shield portion 408, and shield portion 410 is randomly determined.
As the traces in metal shield are randomly spaced and randomly placed, there are portions within planar inductor 400 in which there is no blanket shield disposed between trace and substrate such as location 434, portions in which a metal shield is disposed between trace and substrate such as trace 404, and portions in which a metal shield is partially disposed between trace and substrate such as trace 414.
Portions without metal shield disposed between trace and substrate behave similarly to planar inductor 200 of FIG. 2. Electric fields in these locations terminate on substrate and generate image currents in lossy substrate similarly to planar inductor 200 of FIG. 2. Image currents then cause resistive losses in the substrate, which degrades Q factor of the inductor.
Portions of inductor 400 in which metal traces are disposed between trace and substrate behave similarly to FIG. 3B yet magnitude of eddy current is reduced. Metal shield 402 is slotted and not a solid blanket shield. Due to this, the conductive shield portion is constrained within the traces. Further, the corresponding eddy current loops are constrained within these small traces and do not form a big continuous loop. As such, the eddy current magnitude is reduced as compared to eddy currents within a blanket metal shield. The magnitude of magnetic flux 356 is reduced and degradation of Q factor due to eddy currents in the conductive shield is reduced
In operation, planar inductor 400 contains aspects of planar inductor 200 of FIG. 2. Portions below trace 304 between traces of slotted metal shield 402 behave in a manner similar to planar inductor 200 of FIG. 2, where image currents are induced within substrate 202. Portions below trace 304 directly above traces of slotted metal shield 402 operate in a somewhat different manner, as will be described with additional reference to FIGS. 5A-B.
FIG. 5A illustrates a cross-sectional view of conventional planar inductor 400 with a randomly-traced shield, wherein the cross-section is disposed at a position such that the metal shield includes a centrally slotted shield portion.
As shown in the figure, conventional planar inductor 400 includes a cross sectional portion of trace 304 indicated as a trace portion 502, a cross sectional portion of metal shield 402 indicated as a metal shield portion 506 and substrate 202. Trace portion 502 includes a portion 502A with surface 502B, and a portion 504A with surface 504B. Metal shield portion 506 includes portion 506A with surface 506B and surface 506C and portion 508A with surface 508B and surface 508C.
Additionally shown in the figure are a positive charge 510, an electric field 512, a negative charge 514, a positive charge 516, an electric field 518, a negative charge 520, a negative charge 526, an electric field 528, a positive charge 530, a negative charge 532, an electric field 534 and a positive charge 536. Metal shield portion 506 is disposed between trace portion 502 and substrate 202.
FIG. 5A illustrates the location of charges and current for planar inductor 400 where portion 502A and portion 504A are separated by space 507. As current is applied to trace portion 502, positive charge 510 accumulates on surface 502B causing electric field 512 to flow towards portion 506A. Electric field 512 induces negative charge 514 to accumulate on surface 506B. Opposing positive charge 516 is induced on bottom surface 506C causing electric field 518 to flow toward surface 522, which induces negative charge 520 on substrate surface 522.
Similarly, as the (opposite) current is applied to trace portion 502, negative charge 526 accumulates on surface 504B causing electric field 528 to flow from shield portion 508A. Electric field 528 induces positive charge 530 to accumulate on surface 508B. An opposing negative charge 532 is induced on bottom surface 508C causing electric field 534 to flow from surface 538, which induces positive charge 536 on substrate surface 538.
As mentioned above, current flows through trace portion 502 such that it travels normal and into FIG. 5A at portion 502A and normal and out of FIG. 5A at portion 504A. This current flow is represented by positive charge 510 on surface 502B and by negative charge 526 on surface 504B. This current is mirrored, i.e., as an image current, on substrate 202 in light of the induced negative charge 520 and positive charge 536. Specifically, the image current flows through substrate 202 such that it travels normal and out of FIG. 5A at the surface of substrate 202 below portion 502A and normal and into FIG. 5A at the surface of substrate 202 below portion 504A. Negative charge 520 on substrate surface 522 and positive 536 on substrate surface 538 will be exactly identical to the case of the inductor having no metal shield between the main trace and substrate. So the image current will be of the same magnitude as the case when there is no metal shield between the main trace and substrate. This image current, when flowing in the lossy substrate, creates resistive I2R losses, represented by 540, leading to Q degradation. This Q degradation in this portion of the metal shield will be exactly identical to the case where there is no metal shield between the main trace and substrate. FIG. 5B illustrates a cross-sectional view of conventional planar inductor 400 with a randomly traced shield, wherein the cross-section is disposed at a position such that the metal shield includes an unbalanced slotted shield portion.
As shown in the figure, this portion of conventional planar inductor 400 includes a cross sectional portion of trace 304 indicated as trace portion 550, a cross sectional portion of metal shield 402 indicated as a shield portion 566A, and substrate 202. Trace portion 550 includes portion 552A and portion 554A. Portion 552A includes a surface 552B and portion 554A includes a surface 554B. Shield portion 566A includes a surface 556B, a surface 556C, a surface 556D and a surface 556E.
Additionally shown in the figure are a positive charge 558, an electric field 560, a negative charge 562, a positive charge 564, an electric field 566, a negative charge 568, a negative charge 572, an electric field 574, a positive charge 576, a positive charge 578, an electric field 580, and a positive charge 582.
Shield portion 566A is disposed between substrate 202 and trace portion 550 in an unbalanced distance between portions 552A and 552B.
This portion of conventional planar inductor 400 behaves similarly to conventional planar inductor 400 of FIG. 5A except that there is one shield portion 556A in FIG. 5B rather than two shield portions, 506A and 508A, in FIG. 5A. As current is applied to portion 552A, positive charge 558 accumulates on surface 552B causing electric field 560 to flow towards shield portion 556A. Electric field 560 induces negative charge 562 on surface 556B. Opposing positive charge 564 is induced on surface 556C causing electric field 566 to flow toward surface 570, which induces negative charge 568 on surface 570. For purposes of discussion, in FIG. 5B, three positive charges are illustrated as forming on surface 552B and three corresponding negative charges are illustrated as forming on surface 556B. However, only two positive charges are illustrated as forming on surface 556C and two corresponding negative charges are illustrated as forming on surface 570. The loss of a charge is a result of the charges induced by portion 554A, which will now be discussed.
Negative charge 572 accumulates on surface 554B of portion 554A causing a portion of electric field 574 to flow from a surface 584 of substrate 202 and a portion of electric field 574 to flow from a surface 556D of shield portion 556A. The portion of electric field 574 from shield portion 556A induces positive charge 576 on surface 556B. Positive charge 576 effectively cancels a induced negative charge from electric field 560 from portion 552A. It is this cancellation that results in the illustrated two positive charges on surface 556C. The remaining charges induced on surface 584 that are not under shield portion 556A do not affect charges on surface 556C. The portion of electric field 574 from surface 584 induces positive charges 578 and 582.
Since shield portion 556A extends from under portion 552A to portion 554A, electric field 560 and electric field 574 partially cancel, as shown in FIG. 5B. The remaining residual portion of electrical field 566 and electric field 580 create image currents in substrate 202. This image current, when flowing in the lossy substrate, creates resistive I2R losses, represented by 586, leading to Q degradation. This degradation in Q will be less than the case where there is no metal shield between the inductor and substrate.
In the places of planar inductor 400 where there is no shield located between trace 304 and substrate 202, the electric fields generated by trace 304 induce charges on the surface of substrate 202. In the places of planar inductor 400 where there is a balanced shield but not extending from a positive side to a negative side of planar inductor 400, located between trace 304 and substrate 202, for example as discussed above with reference to FIG. 5A, the electric fields generated by trace 304 induce charges on the surface of substrate 202, exactly equal to the case where there is no shield. In the places of planar inductor 400 where there is a unbalanced shield located between trace 304 and substrate 202, for example as discussed above with reference to FIG. 5B, the electric fields generated by trace 304 induce charges on the surface of substrate 202, albeit smaller than the case where there is a no shield. Also since the shield traces are placed extremely close to each other, at high frequencies of operation, the shield traces capacitively couple to each other leading to flow of eddy currents and hence leading to further Q degradation. The combination of instances of these situations produces an overall image current, which negatively affects the Q factor of planar inductor 400.
What is needed is an inductor with a higher Q factor and a variable frequency modulation.